The present invention concerns pulse generators which are powered through movement or motion. Typically generators or dynamos which generate power from movement generate the power in the form of an A.C. signal. Most pulse generators, however, require a D.C. power supply. Thus pulse generators which are powered through movement require the A.C. signal to be converted or rectified to a D.C. signal. The present invention particularly concerns pulse generators which feature a full-wave rectifier circuit which has dynamic bias. This circuit may be used in various types of movement powered pulse generators and is particularly useful in an implantable medical device, such as a pacemaker, as well as in a wristwatch.
An A.C. signal in general is an oscillation of voltage having both positive-and negative-going excursions. Rectification in general involves reversing the polarity of the negative-going excursions of the oscillatory signal so that the resultant signal has only positive excursions of voltage.
Previously several types of rectifier circuits have been developed. Among the most well-known types is a "Graetz bridge" rectifier circuit. A Graetz bridge, comprises an interconnection of four diodes. The diodes used in a Graetz bridge and in other well-known types of rectifier circuits may be of the conventional P-N type or the well-known Schottky type, among others.
An "ideal" P-N diode (i.e., the theoretical diode typically used for the purposes of conceptual circuit design) permits current to conduct in only one direction (the "forward" direction) and completely prevents the conduction of current in the opposite ("reverse") direction. In addition, it is often acceptable for the purposes of conceptualizing a circuit to assume that there is no voltage drop across an ideal diode. Actual diodes, however, may not completely prevent reverse current (i.e., there may be some "reverse leakage" through a diode.) Actual diodes also typically have a threshold voltage (sometimes called a "turn-on" voltage) of 0.7 volts or so. This means that a forward-bias voltage of at least 0.7 volts must be applied to the diode before forward conduction of current through the diode will commence, and that when current is being conducted through the diode, there will be a 0.7 volt voltage drop across the diode.
Schottky diodes have a relatively lower threshold voltage as compared with P-N diodes. Schottky diodes, however, also tend to have a relatively higher reverse leakage (sometimes also called self-leakage) as compared with P-N diodes.
In many applications, the threshold voltage and reverse leakage characteristics of either P-N diodes or Schottky diodes have negligible impact upon the performance of the circuits in which they are used. In some circumstances, however, the threshold voltage and reverse leakage behavior of a diode can be critical to the operation of a circuit. This is true, for example, in relatively low voltage applications, such as in circuits powered with a supply voltage on the order of one to three volts or lower. Such circuits are found in such pulse generators as a movement powered medical pulse generator, such as a pacemaker, as well as in a wristwatch.
One method used to overcome the threshold voltage problems of conventional diodes involves continuously biasing the diodes to conduction. This is proposed, for example, in U.S. Pat. No. 4,533,988 to Daly et al., entitled "On-Chip CMOS Bridge Circuit." This solution may be unacceptable, however, for applications in which minimizing power supply current drain is of concern. For example, the operational longevity of battery-powered electronic devices can be adversely affected by the current drain associated with continuous biasing circuitry.
The performance of conventional P-N or Schottky diodes may also present complications in relatively high-frequency applications, such as in rectifiers for rectifying A.C. signals in the kHz or higher frequency range. The above-noted continuous biasing arrangement can sometimes improve the diode frequency response, but again, this may not be acceptable in circumstances where current drain on the power supply must be minimized.
Low-impedance full-bridge rectifiers utilizing field-effect transistors (FETs) have been proposed in the prior art. One such rectifier utilizes two cross-connected N-type FETs and two cross-connected P-type FETe to accomplish full-wave rectification. See, e.g., 1984 Siliconix Inc. MOSPOWER Applications Handbook, pp. 5-91-5-92. Such a configuration, however, is primarily useful only where an input voltage is continuously present.